A. Field of the Invention
The invention relates to electronic switches and, more particularly, to high voltage, fast acting semiconductor electronic switches.
B. Prior Art
Numerous devices are presently available for voltage switching applications. For example, devices such as siliconcontrolled rectifiers are capable of switching from a high impedance state to a low impedance state on application of a control signal thereto. However, the switching speed of such devices is limited. Further, once the SCR is turned on, the polarity of the voltage it is switching must be reversed before it can be turned off again. Thus, it is unsuitable for DC switching.
Transistors have also been used as voltage switching devices, but these also suffer numerous disadvantages. To begin with, the breakdown voltage of transistors of moderate cost is limited. Thus, when higher switching voltages are sought from a transistor, several transistors are generally connected in series. This increases the size and cost of the switch, and also places stringent limitations on the driving circuitry, which further increases the cost of the overall switch.
PIN diodes are semiconductor devices having an intrinsic region of semiconductor material (generally silicon or germanium) interposed between regions of P-type and N-type semiconductor material, respectively. Typically, the P and N regions are heavily doped, while the intrinsic (I) region is very lightly doped and is only approximately intrinsic. The equivalent resistance of the diode arises mainly from the intrinsic region, and is a function of the forward current through the diode; its depletion-layer capacitance is essentially constant over a wide range of reverse bias voltage. Because of these characteristics, the PIN diode is frequently used as a variable attenuator, particularly in high frequency (microwave) circuits. It is also commonly used as a switch at microwave frequencies by varying the reverse bias voltage across it; a common switching application is in a microwave ring modulator.
The PIN diode has heretofore been used as a photodiode, that is, a device which generates a current in accordance with the light flux incident on it. Typically, the light flux is incident on the P or N region, and traverses these narrow regions, to the intrinsic region where it creates hole-electron pairs. It has also been suggested that the PIN diode may be used as a photovoltaic device, that is, a voltage generator, by stacking a number of them in series and illuminating the intrinsic region to thereby generate a voltage across the diode. However, it has not heretofore been recognized that the photoresponsive nature of this diode, together with its possession of a substantial region of intrinsic material, make it eminently suitable for use as a high speed, high voltage, linear switch.